A varistor is an electrical component in which the resistance decreases markedly as the voltage applied across the device increases. This characteristic makes the device suitable for applications such as protection against overvoltage surges in electrical circuits. Several types of varistor are available, including:
Zener or avalanche diodes which are effective in clamping transients to low voltages but are costly to fabricate for high surge energy applications. PA1 Metal oxide varistors, based on zinc oxide or other metal oxides and fabricated by ceramic processing techniques. These devices are inexpensive to fabricate but operate best at high voltages and are difficult to adapt for low voltage (3 to 30 V) applications.
Various voltage-dependent resistors have been widely used for stabilization of voltage of electrical circuits or suppression of abnormally high voltage surges induced in electrical circuits. The electrical characteristics of such voltage-dependent resistors are expressed by the relation: EQU I=(V/C).sup.n
where V is the voltage across the resistor, I is the current flowing through the resistor, C is a constant corresponding to the voltage at a given current and exponent n is a numerical value greater than 1. The value of n is calculated by the following equation: EQU n=[log.sub.10 (I.sub.2 /I.sub.1)]/[log.sub.10 (V.sub.2 /V.sub.1)]
where V.sub.1 and V.sub.2 are the voltages at given currents I.sub.1 and I.sub.2, respectively. The desired value of C depends upon the kind of application to which the resistor is to be put. It is ordinarily desirable that the value of n be as large as possible since this exponent determines the extent to which the resistors depart from ohmic characteristics.